1. Field of the Invention
The present invention relates to an electrode for a fuel cell and the fuel cell, and more particularly to the fuel cell using carbon nano-fiber as a catalyst carrier and the fuel cell using this electrode.
2. Description of the Related Art
The fuel cells generate few amount of carbon dioxide and, in recent years, have attracted public attention as a power generation technology having few environmental loading.
The fuel cells are usually of a structure of laminating in order a current collector for a cathode/a catalytic layer for the cathode/an electrolyte film of proton conductivity/an anode catalytic layer and/a current collector for the anode.
The catalytic layer for the electrode (the catalytic layer for the cathode or the catalytic layer for the anode) is required to have electronic conductivity between the catalytic layer for the electrode and the current collector and proton conductivity between the catalytic layer for the electrode and the electrolyte film in addition to hold the sufficient amount of catalytic particles in order to obtain a catalytic function. For this reason, heretofore in the past, a catalytic layer of some tens μm has been formed by mixture of conductive particles having a particle diameter of about 50 nm carried with the catalyst and the proton conductor.
In the catalytic layer for the electrode having such a constitution, for example, an electron formed by the catalyst which is located in the vicinity of an electrolyte film does not reach the current collector unless it moves among a plurality of conductive particles. However, because a contact area among conductive particles is small and a proton conductive material exists among the particles as the case may be, electrical resistance between the conductive particles is high. That is, the conventional catalytic layer is low in electronic conductivity between the current collector and the catalyst layer for the electrode, thereby lowering power generation efficiency.
Further, by densifying the catalytic layer, it is possible to enhance electronic conductivity between the current collector and the catalytic layer for the electrode. However, when the catalytic layer is densified, because diffusion of the fuel or an oxidizer into the catalytic layer is lowered, there occurs a problem that the catalytic function of the catalytic particle cannot be sufficiently utilized.
On the other hand, as a technology regarding the catalyst, there has been a report that the carbon fiber is used as a catalyst carrier and the catalytic particle is carried on this carbon fiber surface (E. Theorid et al: Electrochem. Acta., Vol. 38, No.6, P.793(1993)).
When the carbon fiber carried with the catalytic particle is fabricated and the electrode in which this carbon fiber is temporarily deposited on the current collector surface is adopted for the fuel cell, even if a probability of the electron formed in the vicinity of an electrolyte film moving among the particles (among fibers) during its movement to the current collector is lowered , several times of movements among the particles are usually necessary and it is very difficult to sufficiently enhance the electronic conductivity.
Heretofore in the past, the point that counts much in preparing the electrode for the fuel cell is to have a micro-sized catalyst having a large specific surface area contained in high-density so that high output power can be obtained. In order to prepare such an electrode, for example, the catalyst was formed on a carbon particle surface, so that it became possible to have the catalyst carried on the catalyst carbon particle surface of nano order in high density.
In order to improve the output of the fuel cell, it is important not only to improve activity in the above described catalyst, but also to improve diffusion of the fuel or oxygen into the inside of the electrode. The conventional electrode uses a small-sized carbon black having a large specific surface area as a carrier so as to allow as many catalysts as possible to bear. For this reason, it is practically impossible to control pores of the electrode layer with a result that a small-sized dense electrode having small porosity and pore-size comes out.
That is, though the amount of the catalyst per unit volume increases, because porosity and pore-size in the electrode are small, it becomes difficult for oxygen and the fuel to be supplied into the inside of the electrode, and there occurs a problem that service efficiency of the catalyst which exists inside the electrode is lowered.
Furthermore, it is necessary for the electrode to have a conductive function for the electron generated by the catalyst or catalytic reaction, and from this viewpoint, the electrode using the conventional carbon black as the carrier is required to be made dense. Nevertheless, when the electrode is made dense as described above, diffusion of the fuel or oxygen into the inside of the electrode becomes poor.
In other words, in the case where the conventional carbon black is used as a carrier, it is difficult to realize an electrode, which satisfies three points such as high density of micro-catalyst, good diffusion of fuel and maintenance of an electronic conductive bus at the same time.
For example, it is described in Guangli Che (Nature Vol.393, p346 (1998) that a carbon nano-tube is fabricated by using a template method and, by using this carbon nano-tube as a carrier, a minute catalyst is carried on the inner wall of the nano-tube.
However, because ununiformity of the catalyst toward the longitudinal direction of the nano-tube is recognized, this method is not sufficient enough to realize highdensity of the catalyst.
Although it is possible to make the electrode dense and enhance catalytic density by making the carbon nano-tube minute, diffusion of fuel and the like becomes poor. When a relatively large-sized carbon nano-tube is used, though diffusion of fuel and the like is improved, catalytic density is lowered.
On the other hand, when the anode electrode has high porosity and large-sized pores so as to improve diffusion of fuel, the supplied fuel passes through the anode electrode and further through the proton conductive film to reach the cathode electrode, and the fuel and oxygen end up to directly react there, thereby lowering power generation efficiency.
For this reason, it is necessary for the anode electrode not only to improve diffusion of the fuel, but also to lower permeability of the fuel. While it was not possible for the conventional anode electrode to enhance diffusion of the fuel and at the same time to lower permeability of the fuel.
As described above, because the conventional electrode to be used for the fuel cell is unable to enhance catalytic density and at the same time to enhance diffusion of the fuel, it was unable to develop high output of the fuel cell. Or, in the case of the anode electrode, it was not possible for the anode electrode to enhance diffusion of the fuel and at the same time to lower permeability of the fuel. Further, it was difficult for the conventional fuel cell electrode to sufficiently enhance conductivity of the catalytic layer and therefore to sufficiently enhance power generation efficiency of the fuel cell.
The prevent invention has been made in view of such problems and it is an object of the present invention to provide a fuel cell capable of outputting high power or an electrode for a fuel cell which makes it possible to output high power of the fuel cell. Further, it is an object of the present invention to provide the fuel cell having high power generation efficiency, the electrode for the fuel cell to achieve it and a manufacturing method of the electrode for the fuel cell to achieve it.